1. Field of the Invention
The invention relates to a method for the continuous manufacture of a twin-wall pipe having a pipe socket and consisting of a smooth internal tube and an external tube, which is welded together with the internal tube and which is provided with transverse grooves, and to an apparatus for putting this method into practice.
2. Background Art
A method and an apparatus of the generic type are known from U.S. Pat. No. 5,320,797. During the expansion of the external tube and the internal tube to form a pipe socket, the external tube and the internal tube are extruded more thick-walled than during the production of the twin-wall pipe provided with transverse grooves by the advancing speed of the half shells, which are on the molding path and combine to form molds, being reduced.
Similar methods are known from EP 0 385 465 B1 and WO 88/05377. Reduction of the take-off speed, i.e. the rate at which the internal tube and the external tube are taken off the extruder, does not always have the desired results. Due to the distance, conditioned by construction, of the internal die from the external die, any acceleration or deceleration of the molds results in that the desired modifications of wall thickness occur at places of the twin-wall pipe that may vary in the conveying direction. This interferes with the manufacturing process and also affects the twin-wall pipe negatively, in particular in the vicinity of the pipe socket. Furthermore, due to the predominantly smooth socket geometry, the melt flow for the external tube leads to increased wall thickness in the vicinity of the socket as compared to the area provided with transverse grooves. The contrary is true for the internal tube, which must be stretched beyond the outside diameter of the twin-wall pipe in the vicinity of the pipe socket. In the case of some materials, pipe diameters and wall thicknesses, these facts give rise to considerable drawbacks in the molding process.
It is an object of the invention to embody the method of the generic type such that the internal tube and the external tube each have the desired wall thickness during the production of the pipe socket, and to specify an apparatus for putting the method into practice.
According to the invention, this object is attained in a method of the generic type by the features which consist in extruding a mass flow of melt of plastic material per unit of time and forming an external tube having an inside and an outside; providing the external tube with a corrugation of transverse grooves and corrugation troughs by an overpressure acting from the inside to the outside; extruding a second mass flow of melt of plastic material per unit of time and forming an internal tube into the external tube; pressing the internal tube against the corrugation troughs of the external tube, thereby welding together the internal tube with the external tube; expanding panding the external tube at given intervals and by the application of the overpressure acting from the inside to the outside to form a substantially smooth-walled, approximately cylindrical pipe socket; and modifying at least one of the first and second mass flow of melt of plastic material per unit of time independently of the other one of the second and first mass flow of melt of plastic material per unit of time during the forming of the pipe socket. Due to the fact that at least one melt flow, i.e. the melt volume per time unit, and consequently also the quantity of melt per time unit at a constant temperature, i.e. the mass flow rate, can be modified, optimization of the wall thicknesses of the internal tube and external tube can be attained without any problems over the course of the pipe socket. As a rule, the melt flow for the external tube is maintained or reduced during the manufacture of the pipe socket and the melt flow for the internal tube is increased in order that a sufficient wall thickness be available for the stretching of the internal tube at the beginning of the socket. In the method according to the invention, it is possible without any problems to run a corresponding meltflow curve by corresponding triggering.
An apparatus which is suitable for putting the method into practice is specified by the features wherein half shells are arranged on a machine bed for circulation and guidance in a conveying direction, which half shells are provided with annular mold recesses and which combine in pairs on a molding path to form a mold with a central longitudinal axis; wherein at least one pair of half shells is provided with a socket recess; wherein means are provided for the production of relative overpressure acting from the inside to the outside; wherein a crosshead of a first extruder is disposed upstream of the molding path; wherein the crosshead is provided with an outer die for the extrusion of an external tube, and, downstream in the conveying direction, with an inner die for the extrusion of an internal tube, and, at a downstream end in the conveying direction, with a calibrating mandrel; wherein the inner die is connected to the first extruder; wherein a second extruder is provided; wherein the second extruder is connected to the outer die; wherein an extrusion rate of at least one of the first and second extruder is modifiable; and wherein switches are provided, by means of which the extrusion rate of said at least one of the first and second extruder is modified while the socket recess moves over one of the dies.
Further features, advantages and details of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing.